This invention relates to an ion implantation beam monitor, in particular for monitoring the quality or stability of an ion beam in an ion implanter which implants ions into substrates such as semiconductor wafers.
This application claims priority under 35 U.S.C. xc2xa7119 of United Kingdom Application No. 9814285.4, which has a filing date of Jul. 1, 1998, in accordance with 37 C.F.R. xc2xa71.55(1)(ii) and which was identified in accordance with the regulations under the Patent Cooperation Treaty as a priority document to PCT International Application No. PCT/GB99/02087 from which this application is derived.
Semiconductor devices are typically formed from a semiconductor substrate material into which atoms or molecules of selected dopants have been implanted or defused. The dopant particles produce regions in the semiconductor substrate having varying conductivity. By selecting appropriate dopant materials, the majority charge carrier may be locally altered within the substrate.
One preferred technique for adding dopant materials to semiconductor substrates uses ion implantation. This technique minimises the size of the device features created by the dopants within the substrate, reducing the overall size of the semiconductor device itself and increasing operational speed.
The principles of operation of an ion implantation apparatus will be familiar to those skilled in the art. Briefly, a plasma generates positive ions of the selective dopant material in an ion source. The required positively charged ions are extracted from the ion source, which are accelerated by application of an acceleration potential through a magnetic field. The magnetic field is generated by a mass selection arrangement which bends the extracted ions around a curved path. The radius of curvature of the flight path of the ions is dependent upon the mass/charge ratio of the individual ions. The exit of the mass selection arrangement has a slit within it so that only ions having a predetermined mass/charge ratio can exit the mass selection arrangement.
Those ions exiting the mass selection arrangement impinge upon a semiconductor substrate to be doped. Typically, this substrate will have previously been patterned with photo resist so that only selected regions are doped.
The depth to which ions are implanted in the substrate will depend upon their energy when incident upon the substrate. In order to generate ions for implantation, it is standard practice to provide an arrangement to generate a desired potential difference between the xe2x80x9cflight tubexe2x80x9d and sample holder. The flight tube is an expression used throughout this specification and the claims to refer to the metalwork of the mass selection arrangement which is normally all at a common potential with reference to earth. However, the term xe2x80x9cflight tubexe2x80x9d is also applicable to any structure where corresponding electrode arrangements arisexe2x80x94for example a structure which itself has a potential referenced to earth and from which the potential of the ion source is in turn referenced. For example, the flight tube may contain an r.f. accelerator or booster positioned downstream of the mass selector for which the final exit electrode of the booster is at a potential referenced to the mass selection.
Usually, the target substrate is held at or near earth potential. If the flight tube is at a positive potential with respect to the sample holder/target substrate, then the ions are accelerated post mass selection. This is called xe2x80x9cpost accelerationxe2x80x9d, and is desirable when deep implantation of ions is required. On the other hand, if the flight tube is at a negative potential with respect to the sample holder, then the ions are decelerated post mass selection. This is called xe2x80x9cpost decelerationxe2x80x9d. Post deceleration is typically employed to produce very shallow structures within the surface of the substrate, to decrease the size and increase the speed of the resultant semiconductor device.
In some arrangements, it is desirable to operate the flight tube at substantially the same potential as the substrate holder, so that there is no post acceleration or deceleration. This mode of operation is termed the xe2x80x9cdrift modexe2x80x9d.
Frequently, the cross-sectional area of the ion beam at the substrate is less than that of the substrate, in order to prevent ions in the beam from sputtering the walls of the apparatus and introducing impurities, and to increase doping uniformity. This necessitates scanning of either the substrate relative to a fixed direction ion beam or scanning the ion beam across a fixed substrate. In practice, it is preferable to scan the substrate while maintaining the ion beam in a fixed direction.
It is important when doping substrates with an ion beam from an ion implantation apparatus that the ion current striking the substrate be monitored. This is to ensure correct doping. If, for example, the ion beam contains a drop-out, then the substrate will not be doped properly. Several ways of monitoring the beam have been proposed, and a Faraday detector, such as a Faraday cup or bucket, is often employed.
In one known apparatus, the Faraday detector acts as a beam stop arranged down stream of a rotating spoked wheel. The rotating wheel carries a plurality of substrates to be doped and is scanned across an ion beam. The beam stop collects ions passing through or missing the wheel, as well as any secondary electrons generated. In another apparatus, a solid wheel is employed instead, and the Faraday detector is arranged upstream of the wheel.
Each of these ion beam monitors suffer from drawbacks, however. It is therefore an object of the present invention to provide an improved apparatus and method for monitoring the ion beam.
According to a first aspect of the present invention, there is provided an ion implantation apparatus comprising a holder for a substrate to be implanted, a source of ions to be implanted in the substrate, a flight tube through which ions from the source travel towards the holder, an ion accelerator arranged to supply an acceleration bias between the source and the flight tube such that the ions are accelerated to an acceleration energy, a power supply arranged to generate a desired potential difference between the substrate holder and flight tube, an electrically conductive return current path connected to conduct the entirety of the current returned to the flight tube such that the flight tube is maintained at a desired potential relative to the substrate holder, and a return current monitor arranged to provide a signal representative of the return current flowing through the return current path.
Measuring the total current returned to the flight tube provides a useful indication of the total ion current in the beam leaving the flight tube, as well as any electrons travelling back to, and being absorbed by, the flight tube. This in turn permits the quality of the ion beam post mass selection to be monitored, continuously if desired. Previously, for example when using a beam stop behind the substrate holder, the ion beam could only be monitored (via the beam stop current) when the ion beam was not absorbed by the substrate holder. This prevented drop-outs in the ion beam, for example due to arcing in the ion source, from being detected if these occurred whilst the substrates were absorbing the ion beam rather than the beam stop.
Preferably, the power supply generates a potential difference between the substrate holder and flight tube such that the ions are decelerated between the flight tube and the substrate holder. Although post mass selection acceleration is desirable in certain cases, it is more often preferable to decelerate the ions prior to impact with a substrate.
According to a second aspect of the present invention, there is provided an ion implantation apparatus comprising a holder for a substrate to be implanted, a source of ions to be implanted in the substrate, a flight tube through which ions from the source travel towards the holder, an ion accelerator arranged to supply an acceleration bias between the source and the flight tube such that the ions are accelerated to an acceleration energy, a low resistance, electrically conductive return current path connected to conduct the entirety of the current returned to the flight tube such that the flight tube is maintained at substantially the same potential as the substrate holder, and a return current monitor arranged to provide a signal representative of the return current flowing through the return current path.
Connecting the flight tube to the substrate holder via a low resistance path means that the ions exiting the flight tube are neither accelerated nor decelerated as they approach the substrate holder.
Preferably, the ion implantation apparatus of the first and second aspects of the present invention further comprises a beam stop arranged to absorb a portion of the ion beam not absorbed by the substrate or substrate holder, and to generate a beam stop current therefrom, the beam stop current being returned to the flight tube via the return current path. In that case, the apparatus may further comprise a comparator arranged to provide a signal representative of the difference between the return current through the return current path and the beam stop current. Thus, the beam stop current may be compared, in real time, with the total current being returned from the substrate, substrate holder and beam stop to the flight tube.
According to a third aspect of the present invention, there is provided an ion implantation apparatus comprising a holder for a substrate to be implanted, a source of ions to be implanted in the substrate, a flight tube through which ions from the source travel towards the holder, an ion accelerator arranged to supply an acceleration bias between the source and the flight tube such that the ions are accelerated to an acceleration energy, a beam stop arranged to absorb a portion of the ion beam not absorbed by the substrate or substrate holder, and to generate a beam stop current therefrom, an electrically conductive controlled current path connected to conduct the entirety of the current to the flight tube such that a desired potential difference is maintained between the substrate holder and flight tube, and a comparator arranged to provide a signal representative of the difference between the controlled current through the return current path and the beam stop current.
Comparing the beam stop current with the total current returned to the flight tube allows a cross check on beam stop accuracy to be made, when the ion beam does not impinge upon any part of the substrate holder.
The apparatus may also include scanning means for scanning the ions relative to the substrate, and optionally relative to the beam stop as well. For example, the substrate may be scanned relative to a fixed ion beam direction.
Preferably, the substrate holder comprises a plurality of substrate supports each mounted relative to a rotatable hub. The beam stop may include a Faraday-type detector.
The invention also extends to a method of implanting ions in a substrate at a desired implant energy, comprising accelerating ions to a transport energy by supplying an acceleration bias between an ion source and a flight tube through which the ions travel, transporting the ions through the flight tube to the substrate, generating a return current signal representative of the entirety of the current returned to the flight tube, the return current being controlled such that a desired potential difference is maintained between the substrate holder and flight tube, and monitoring the ions transported through the flight tube to the substrate based upon the signal representative of the return current.
Furthermore, there is provided a method of monitoring the quality of an ion beam in an ion implantation apparatus, comprising accelerating ions to a transport energy by supplying an acceleration bias between an ion source and a flight tube through which the ions travel, transporting the ions as in an ion beam through the flight tube to a substrate, generating a return current signal representative of the entirety of the current returned to the flight tube, the return current being controlled such that a desired potential difference is maintained between the substrate holder and flight tube, and monitoring the quality of the ion beam based upon the signal representative of the return current.